The present invention relates to an interferometric measuring device for detecting the shape or distance, in particular of rough surfaces. The interferometric measuring device has at least one spatially coherent beam gun unit, whose beam in a measuring probe is divided into a reference measuring beam guided through and reflected in a measuring reference arm and a measuring beam guided through a measuring arm and reflected on the rough surface; a device for modulating the light phase or for shifting the light frequency (heterodyne frequency) of a first partial beam with respect to the light phase or the light frequency of a second partial beam using a superposition unit for superimposing the reflected measuring beam on the reflected measuring reference beam; a beam splitting unit and receiving unit for splitting the superimposed beam into at least two beams having different wavelengths and converting the beams into electrical signals; and an analyzer, in which the shape or distance of the rough surface can be determined on the basis of a phase difference of the electrical signals.
Such an interferometric measuring device is referred to in European Patent No. 126 475. In this measuring device, rough surfaces of a measured object are measured interferometrically. A beam gun unit that has laser light sources, which emit light of different wavelengths, is used. The laser light is divided into a reference beam of a reference beam path and a measuring beam of a measuring beam path using a beam splitter. The measuring beam path impinges on the surface to be measured, while the reference beam path is reflected on a reference surface, for example in the form of a mirror. The light reflected from the surface and the reference surface is combined in the beam splitter and focused, with the help of a lens, in a interferogram plane, where a speckle pattern is obtained. This speckle pattern is analyzed to determine the surface shape, a phase difference of the interferogram phases in the measuring point being determined. In order to simplify the analysis, a heterodyne process is used. The frequency of the reference beam is shifted with respect to the frequency of the measuring beam by a heterodyne frequency using a frequency shifter in the reference beam path. With this measuring device, a fine resolution of the surface shapes can be obtained. The laser light that has different discrete wavelengths can be generated using individual laser light sources such as an argon laser. Such laser light sources are relatively expensive. Semiconductor lasers with a plurality of different discrete wavelengths (modes), on the other hand, are unsuitable for such interferometric measurements due to their insufficient stability and the resulting wavelength shift. As an alternative, a plurality of laser light sources such as laser diodes can be used in order to generate the different discrete wavelengths. It is technically difficult to generate the spatial coherence of the beam composed of the different wavelengths. In addition, in such laser diodes the instability of the individual discrete wavelengths is particularly unfavorable. Providing a plurality of different discrete wavelengths is therefore also costly.
In using laser light for generating the discrete wavelengths it is also difficult to accurately set the desired distance between the measuring probe and the surface (autofocus function). The design using the laser light source also makes it difficult to design the measuring part as an easy-to-handle unit which can be used, for example, instead of a mechanical probe of a measuring machine.
Another interferometric measuring device is described in German Patent Application No. 39 06 118, in which optical fibers are provided between a plurality of laser light sources and a measuring section. Here too, a phase difference is evaluated for determining the surface structures. This measuring device is also disadvantageous with regard to handling in places that are difficult to access.
An object of an exemplary embodiment of the present invention is to provide a heterodyne or phase interferometric measuring device, with which very accurate measurements of surface shapes and surface distances are possible under industrial conditions even on relatively difficult-to-access surfaces such as small boreholes, and which is easy to handle and has a simple design.
The object is achieved with the exemplary embodiment of the present invention, according to which the beam emitted by the beam gun unit is broad-band and has a short coherence time.
Surprisingly, the broad-band, short coherence time beam gun units that have a higher spatial coherence than light sources of a heterodyne interferometric measuring unit, in particular in conjunction with the measurements of rough surfaces, are not only well-suited, but also offer considerable advantages compared to laser light sources. The spatial coherence of the beam naturally results from the light source. Instabilities of the spectral beam distribution of the light source have virtually no effect on the measurements, since not only are individual fixed wavelengths always selected using the beam gun unit (e.g. grating) and the assigned beam receiving unit from the continuous spectrum in a stable manner, but, in particular, also their difference, which is important for accurate and unambiguous evaluation, is preserved in a stable manner. Changes in the intensity of the wavelengths in the case of instabilities have no effect due to the heterodyne technology, since in this case only the phases are relevant. The short time coherence beam allows an autofocus function to be implemented in a very simple manner, since the heterodyne signal is present only for a certain distance range determined by the short coherence length between the measuring part and the surface. Furthermore the short coherence length offers the advantage that, using coherence multiplexing, the entire measuring system can be divided into a modulation interferometer containing the active components and a small, sturdy, and easy-to-handle measuring part designed as a measuring probe, separated spatially therefrom by optical fibers, for example.
An exemplary embodiment of the present invention also includes an embodiment that is advantageous for use in manufacturing. In the Mach-Zehnder embodiment, the difference of angular dispersion is minimized due to the two acoustical-optical modulators arranged in the two partial beam paths.
If the beam gun unit has a short coherent, broad-band additional light source, which can be operated for light amplification or as a backup light source, the light intensity can be enhanced by using both light sources. As an alternative, the additional light source can be used as a backup light source in case of failure of the other light source.
The measures of an additional device for frequency shifting is arranged in the beam path of the second partial beam for frequency shifting the first partial beam with respect to the second partial beam, and the device and the additional device for frequency shifting are acoustical-optical modulators are suitable for achieving a low angular dispersion. The arrangement of modulators in the two beam paths reduces measurement errors due to temperature drifts and the related change in the refractive index of an acoustical-optical modulator, which would result in undesirable phase shifts.
Furthermore, the fact that the beam splitting and receiving. unit is a spectral device that has a downstream photodetector matrix and the beam splitting and receiving unit also mounted in the unit and is coupled to the measuring probe via the optical fiber arrangement, is favorable for the design and for evaluation.
Design and evaluation are further facilitated by the fact that the measuring probe, including the measuring arm, the measuring reference arm, and a beam splitter of the measuring probe, is designed as a Michelson or Mirau interferometer and that an optical path difference produced in the measuring arm and in the measuring reference arm compensates for the optical path difference produced by the time delay element.
An additional beam path is formed starting from the second beam splitter, leading to a reference probe that has a reference probe reference arm and a reference probe measuring arm. An additional beam splitting and receiving unit is provided in the unit, and the unit is coupled to the reference probe via an additional optical fiber arrangement, so that an error of the rotating table used for moving the measuring object having the surface structure to be measured can be compensated. Furthermore the reference probe can be used for compensating a drift of the modulation interferometer provided in the unit, caused, for example, by temperature.